Where to hang it?

Grouping;
Hanging pictures with a similar theme together in the same room/space. Some theme ideas are; family photos; landscapes; figurative; black and whites. Aviod hanging pictures randomly around your house or office, try and tell a story.

Placement;
Don't hang them too high! High pictures are intimidating. The centre of the picture should be around eye level or if your short/tall 1.5m.
Pictures should have the same centre height. Don't hang them so that the top of the pictures are the same and don't match the top with a door or a window or a cupboard.
If you want to hang something above a couch then you will need to have someone hold the picture for you to judge the height. If there are other pictures in the same room hang those pictures first before you decide how high to hang.
Don't hang a picture on every wall. Leave some vacant space. This gives you some peace and leaves the imagination open.

How to hang it?

Mark the wall lightly in pencil when you've measured the spot to put the hook.
To make a hook in the wall use a cap head screw.
Be sure above a bed.

Two hooks or one hook?
For small pictures (less than50 x50cm) one hook is fine. Use blue tak in the bottom corners to steady it if needed.
For larger pictures (greater than 50 x50cm) two hooks is better. You can put two hooks for the string (simple), or use two d-latches carefully attached at the same height on each side of the frame and place two hooks level in the wall to match these d-latches (more difficult). The advantage of hanging directly on the d-latches is the the picture will never hang crookedly and the stress in the frame is minimised. If you need to adjust the picture slightly after it's hung either tap the hook or move the d-latch.

Measuring;
Measure twice cut once or in this case drill once.

Drilling;
Don't drill directly above power points or light switches the cables run up the wall.

Plaster;
Use a fine drill bit to test what is behind the plaster. If you hit nothing then use a wallmate (see picture). If you hit metal or wood -look at the drill bit - then pre-drill slightly thinner than your screw and then screw straight in. If you hit masonary then see below. Be careful! Behind some plaster walls there is insulation. This stuff will wrap around your drill bit and eventually explode out of the wall. If you feel the drill touch something spongey stop and use a wallmate.
If your hanging something very heavy you might need to find a support beem (stud) for extra support. You can buy a 'stud finder' from the hardware store that will do the job. Usually the stud is not in the right spot for where you want to hang the picture. So hang the picture as above and then use the stud as extra support. Screw a d-latch into the frame in the right postion for the stud.

Masonary - bricks, concrete;
Aim the drill slightly down when drilling into the wall, this means the plug won't be pulled out by the wieght of the frame.
The only problem with bricks is if you hit the mortar (between the bricks) this is especially bad if the wall is very old because the mortar is usually very weak. THe problem with the mortar is that the drill 'wanders' and makes a big hole. To minimise this problem hold the drill firmly and only push gently to make the hole. This will stop the drill 'wandering'. Sometimes you'll still make a bigger hole than you wanted. Don't worry just hammer in a bigger wall plug and drill deeper.

Tips
If you need to adjust the height of the picture after you have put a screw in then it's easier to move the string on the back of the frame rather than drill a new hole.
When drilling hold a dust pan under the drill to catch the falling dust. Your wife will love you.
To stablize the picture, especially when hanging off one hook use blue tak in the corners.